Implement writing narrow strings into a wide writer

test/format-test.cc

@@ -325,6 +325,7 @@ TEST(WriterTest, WriteWideChar) {
 
 TEST(WriterTest, WriteString) {
   CHECK_WRITE_CHAR("abc");
+  CHECK_WRITE_WCHAR("abc");
   // The following line shouldn't compile:
   //MemoryWriter() << L"abc";
 }